With my T7300 2.0GHz and Kingston V100 64gb SSD the results are
Bitlocker off → on
Sequential read 243 MB/s → 140 MB/s
Sequential write 74.5 MB/s → 51 MB/s
Random read 176 MB/s → 100 MB/s
Random write, and the 4KB speeds are almost identical.
Clearly the processor is the bottleneck in this case. In real life usage however boot time is about the same, cold launch of Opera 11.5 with 79 tabs remained the same 4 seconds all tabs loaded from cache.
A small build in VS2010 took 2 seconds in both situations. Larger build took 2 seconds vs 5 from before. These are ballpark because I’m looking at my watch hand.
I guess it all depends on the combination of processor, ram, and ssd vs hdd. In my case the processor has no hardware AES so compilation is worst case scenario, needing cycles for both assembly and crypto.
A newer system with Sandy Bridge would probably make better use of a Bitlocker enabled SDD in a development environment.
Personally I’m keeping Bitlocker enabled despite the performance hit because I travel often. It took less than an hour to toggle Bitlocker on/off so maybe you could just turn it on when you are traveling then disable it afterwards.
Thinkpad X61, Windows 7 SP1